1 66 ARTHUR G. VESTAL. 



to be found in extensive areas representing this habitat, its 

 frequency for that habitat was considered to be high. From the 

 records which thus accumulated it was possible to arrive at a 

 relative estimate of frequency and abundance for each species in 

 its various habitats. Though the method is not free from error, 

 it is the best which was available, and it is believed to be not far 

 from a representation of actual conditions. Terms expressing 

 degrees of frequency and abundance are dominant, abundant, 

 frequent, occasional, infrequent, and accidental. These terms are 

 used in the table of distribution on p. 153, and are there defined. 



The Relation of the Animal to Plant and Animal Communities. 



The Plant Association as an Index of the Habitat. The area of 

 the habitat as considered in this study has already been defined 

 as the area conveniently marked by the extent of the plant asso- 

 ciation. In the plant association or plant community physical 

 conditions and vegetation are generally uniform. In the early 

 stages of development of vegetation, local physical conditions 

 dominate. In later stages the vegetation assumes the type 

 determined by climatic conditions, and exerts nearly com- 

 plete control over local physical factors. Thus the grass- 

 hoppers in early stages of vegetation, as the CEdipodincB, most 

 of which live in very open grassland, are more intimately 

 associated with physical conditions, which to them are more 

 important, while those of advanced stages of vegetation de- 

 pend more upon vegetational factors, and less upon the char- 

 acter of the soil. Melanoplus islandicus, associated with climax 

 deciduous forest, is a species of the second type. The grass- 

 hoppers as a group are most abundant in early stages of vegeta- 

 tion in forest climates, while in grassland climates they occur 

 abundantly in all stages. The plant association may thus be 

 taken as the index of environmental conditions; it expresses the 

 resultant of physical and vegetational conditions to which animals 

 are subjected. Within the area of the plant association local 

 variabilities in physical conditions are usually accompanied by 

 local variabilities in the vegetation; the latter may exist inde- 

 pendently. Grasshopper species are affected by these local 

 differences. 



